Internal combustion engines can produce byproducts of the fuel combustion process, including various oxides of nitrogen, referred to collectively herein as NOx gases. Exhaust gas treatment systems can be used in vehicles to treat the NOx gases created in the combustion process.
Exhaust gas treatment systems generally include a selective catalytic reduction (SCR) device to reduce NOx gases. The SCR device uses a reductant capable of reacting with NOx gases to convert the NOx gases into inert byproducts, i.e., nitrogen and water. For example, the reductant can be an aqueous solution of urea, which is injected into the engine's exhaust stream. Once the reductant is in the exhaust stream, the reductant is absorbed into a catalyst of the SCR device, where the catalytic action of the SCR device ultimately converts NOx gases into the inert byproducts.
Exhaust gas treatment systems also include a diesel particulate filter (DPF) to filter out particles or particulate matter in the exhaust stream that is emitted by the engine. Generally, the DPF captures or traps sooty particulate matter and other suspended particulate matter from the exhaust stream. For example, the particulate matter can include carbonaceous soot particulates that can be oxidized to produce gaseous carbon dioxide, as well as other non-combustible particulates (i.e., ash) that are not capable of being oxidized.
Generally, the SCR device is spaced from the DPF such that the SCR device and the DPF are separate and independent components. Therefore, the SCR device converts NOx gases into the inert byproducts independently of the particulate matter being trapped by the DPF.
In-situ thermal regeneration of the DPF can be conducted periodically to burn off the accumulated particulate matter. However, thermal regeneration cannot remove ash from the DPF, and therefore, ash continues to accumulate in the DPF throughout the life of the DPF.